Fuel injector

ABSTRACT

A lean burn fuel injector comprises a fuel feed arm and a fuel injector head. The fuel feed arm has a single pilot fuel supply passage and a single main fuel supply passage extending there-through. The fuel injector head has a coaxial arrangement of an inner pilot air-blast fuel injector and a coaxially arranged outer main air-blast fuel injector. The inner pilot air-blast fuel injector comprises a coaxial arrangement of a pilot inner air swirler passage and a pilot outer air swirler passage. The outer main air-blast fuel injector comprises a coaxial arrangement of a main inner air swirler passage and a main outer air swirler passage. The single main fuel supply passage is arranged to supply main fuel to the main inner air swirler passage. The single pilot fuel supply passage is arranged to supply pilot fuel onto the pre-filming surfaces in the pilot inner and outer air swirler passages.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is based upon and claims the benefit of priority from British Patent Application No. GB 1720167.4, filed on 4 Dec. 2017, the entire contents of which are incorporated by reference.

BACKGROUND Technical Field

The present disclosure concerns a fuel injector and in particular to a lean burn fuel injector fora gas turbine engine combustion chamber.

Description of the Related Art

A current lean burn fuel injector comprises a fuel injector head having a coaxial arrangement of an inner pilot air-blast fuel injector and an outer main air-blast fuel injector and the outer main air-blast fuel injector is arranged coaxially radially outwardly of the inner pilot air-blast fuel injector. The inner pilot air-blast fuel injector comprises a coaxial arrangement of a pilot inner air swirler passage and a pilot outer air swirler passage. The outer main air-blast fuel injector comprises a coaxial arrangement of a main inner air swirler passage and a main outer air swirler passage. A main fuel supply passage is arranged to supply main fuel to the main inner air swirler passage and a pilot fuel supply passage is arranged to supply pilot fuel into the pilot inner air swirler passage.

In an annular combustion chamber using lean burn fuel injectors of the current arrangement the pilot fuel interacts with the air in the pilot inner air swirler passage and the main fuel interacts with the air in the main inner air swirler passage. The pilot fuel and air mixture exiting from the pilot inner air swirler passage is shielded from the main fuel and air mixture exiting from the main inner air swirler passage by the air exiting the pilot outer air swirler passage. The fuel and air mixture from the pilot inner air swirler passage tends to flow directly downstream along the centre line of the annular combustion chamber. The fuel and air mixture from the main inner air swirler passage tends to flow radially outwardly and radially inwardly towards the radially outer and radially inner annular walls of the annular combustion chamber. The air from the pilot outer air swirler passage provides a recirculating feature to couple the flows from the pilot inner air swirler passage and the main inner air swirler passage.

The present disclosure seeks to provide an improved fuel injector.

SUMMARY

According to a first aspect of the present disclosure there is provided a fuel injector comprising a fuel feed arm and a fuel injector head, the fuel feed arm having a fuel supply passage extending there-through, the fuel injector head having an air-blast fuel injector, the air-blast fuel injector comprising, in order radially outwardly, a coaxial arrangement of an inner air swirler passage and an outer air swirler passage, the inner air swirler passage and the outer air swirler passage being arranged immediately adjacent each other, a downstream end of an annular member defining the radially outer extremity of the outer air swirler passage being frustoconical, the downstream end of the annular member defining the radially outer extremity of the outer air swirler passage being convergent in a downstream direction, the fuel supply passage being arranged to supply a first portion of the fuel into the inner air swirler passage and to supply a second portion of the fuel into the outer air swirler passage, the fuel supply passage being arranged to supply fuel onto a pre-filming surface in the inner air swirler passage, the fuel supply passage being arranged to supply fuel onto a pre-filming surface in the outer air swirler passage.

Note that the inner air swirler passage and the outer air swirler passage being immediately adjacent each other means that there are no other air swirler passages arranged coaxially between the inner air swirler passage and the outer air swirler passage.

The fuel injector may be a rich burn fuel injector, the fuel injector having an additional air swirler passage arranged coaxially around the outer air swirler passage.

The fuel injector may be a lean burn fuel injector the fuel feed arm having a single pilot fuel supply passage extending there-through and a single main fuel supply passage extending there-through, the fuel injector head having a coaxial arrangement of an inner pilot air-blast fuel injector and an outer main air-blast fuel injector, the outer main air-blast fuel injector being arranged coaxially radially outwardly of the inner pilot air-blast fuel injector, the inner pilot air-blast fuel injector comprising, in order radially outwardly, a coaxial arrangement of a pilot inner air swirler passage and a pilot outer air swirler passage, the outer main air-blast fuel injector comprising, in order radially outwardly, a coaxial arrangement of a main inner air swirler passage and a main outer air swirler passage.

The pilot inner air swirler passage may have a pre-filming surface. The pilot outer air swirler passage may have a pre-filming surface. The main inner air swirler passage may have a pre-filming surface. The main outer air swirler passage may have a pre-filming surface.

The pilot inner air swirler passage and the pilot outer air swirler passage may be arranged immediately adjacent each other, e.g. there are no other air swirler passages arranged coaxially between the pilot inner air swirler passage and the pilot outer air swirler passage. The main inner air swirler passage and the main outer air swirler passage may be arranged immediately adjacent each other, e.g. there are no other air swirler passages arranged coaxially between the main inner air swirler passage and the main outer air swirler passage.

The single main fuel supply passage may be arranged to supply main fuel to the main inner air swirler passage or the main outer air swirler passage, the single pilot fuel supply passage being arranged to supply a first portion of pilot fuel into the pilot inner air swirler passage and to supply a second portion of pilot fuel into the pilot outer air swirler passage, the single main fuel supply passage being arranged to supply main fuel onto a pre-filming surface in the main inner air swirler passage or the single main fuel supply passage being arranged to supply pilot fuel onto a pre-filming surface in the main outer air swirler passage, the single pilot fuel supply passage being arranged to supply pilot fuel onto a pre-filming surface in the pilot inner air swirler passage, the single pilot fuel supply passage being arranged to supply pilot fuel onto a pre-filming surface in the pilot outer air swirler passage.

The single pilot fuel supply passage may be arranged to supply pilot fuel into the pilot inner air swirler passage or the pilot outer air swirler passage, the single main fuel supply passage being arranged to supply a first portion of the main fuel into the main inner air swirler passage and to supply a second portion of the main fuel into the main outer air swirler passage the single main fuel supply passage being arranged to supply main fuel onto a pre-filming surface in the main inner air swirler passage, the single main fuel supply passage being arranged to supply pilot fuel onto a pre-filming surface in the main outer air swirler passage, the single pilot fuel supply passage being arranged to supply pilot fuel onto a pre-filming surface in the pilot inner air swirler passage or the single pilot fuel supply passage being arranged to supply pilot fuel onto a pre-filming surface in the pilot outer air swirler passage.

The single pilot fuel supply passage may be arranged to supply a first portion of pilot fuel into the pilot inner air swirler passage and to supply a second portion of pilot fuel into the pilot outer air swirler passage and the single main fuel supply passage being arranged to supply a first portion of the main fuel into the main inner air swirler passage and to supply a second portion of the main fuel into the main outer air swirler passage, the single main fuel supply passage being arranged to supply main fuel onto a pre-filming surface in the main inner air swirler passage, the single main fuel supply passage being arranged to supply pilot fuel onto a pre-filming surface in the main outer air swirler passage, the single pilot fuel supply passage being arranged to supply pilot fuel onto a pre-filming surface in the pilot inner air swirler passage and the single pilot fuel supply passage being arranged to supply pilot fuel onto a pre-filming surface in the pilot outer air swirler passage.

A pilot fuel passage may be arranged coaxially between the pilot inner air swirler passage and the pilot outer air swirler passage.

The single pilot fuel supply passage may be arranged to supply pilot fuel to the pilot fuel passage.

The pilot fuel passage may be arranged to supply the pilot fuel into the pilot inner air swirler passage and into the pilot outer air swirler passage.

The pilot fuel passage may be arranged to supply pilot fuel onto a pre-filming surface in the pilot inner air swirler passage.

The pilot fuel passage may have a fuel swirler to swirl the pilot fuel supplied onto the pre-filming surface in the pilot inner air swirler passage.

The pilot fuel passage may be arranged to supply pilot fuel onto a pre-filming surface in the pilot outer air swirler passage.

The pilot fuel passage may have a fuel swirler to swirl the pilot fuel supplied onto the pre-filming surface in the pilot outer air swirler passage.

A second pilot fuel passage may be arranged coaxially between the pilot outer air swirler passage and the main inner air swirler passage.

The single pilot fuel supply passage may be arranged to supply pilot fuel to the second pilot fuel passage.

The pilot fuel passage may arranged to supply the pilot fuel into the pilot inner air swirler passage and the second pilot fuel passage is arranged to supply pilot fuel into the pilot outer air swirler passage.

The pilot fuel passage may be arranged to supply pilot fuel onto a pre-filming surface in the pilot inner air swirler passage.

The pilot fuel passage may have a fuel swirler to swirl the pilot fuel supplied onto the pre-filming surface in the pilot inner air swirler passage.

The second pilot fuel passage may be arranged to supply pilot fuel onto a pre-filming surface in the pilot outer air swirler passage.

The second pilot fuel passage may have a fuel swirler to swirl the pilot fuel supplied onto the pre-filming surface in the pilot outer air swirler passage.

The pilot inner air swirler passage and the pilot outer air swirler passage may be arranged to swirl the air in opposite directions.

The fuel may be supplied into the outer air swirler passage through an annular slot. The fuel may be supplied into the inner air swirler passage through an annular slot.

The number of points at which the fuel is supplied into the outer air swirler passage may be the same as the number of swirl vanes in the outer air swirler passage.

The points at which the fuel is supplied into the outer air swirler passage are located adjacent the leading edge of the swirl vanes, adjacent the trailing edge of the swirl vanes or between the leading edge of the swirl vanes and the trailing edge of the swirl vanes.

The points at which the fuel is supplied into the outer air swirler passage may be aligned circumferentially with the leading edge of the swirl vanes, aligned circumferentially with the trailing edge of the swirl vanes or arranged circumferentially between the leading edge of the swirl vanes and the trailing edge of the swirl vanes.

The points at which the fuel is supplied into the outer air swirler passage may be equi-circumferentially spaced.

The mass flow of the fuel to the inner air swirler passage to the mass flow of the fuel to the outer air swirler passage may be between 10:1 and 1:10.

The mass flow of the fuel to the inner air swirler passage to the mass flow of the fuel to the outer air swirler passage may be 1:1.

A main fuel passage may be arranged coaxially between the main inner air swirler passage and the main outer air swirler passage.

An intermediate air swirler passage may be sandwiched between the pilot outer air swirler passage of the inner pilot air-blast fuel injector and the main inner air swirler passage of the outer main air-blast fuel injector.

The main inner air swirler passage and the main outer air swirler passage may be arranged to swirl the air in opposite directions.

According to a second aspect of the present disclosure there is provided a method of operating a fuel injector, the fuel injector comprising a fuel feed arm and a fuel injector head, the fuel feed arm having a fuel supply passage extending there-through, the fuel injector head having an air-blast fuel injector, the air-blast fuel injector comprising, in order radially outwardly, a coaxial arrangement of an inner air swirler passage and an outer air swirler passage, the inner air swirler passage and the outer air swirler passage being arranged immediately adjacent each other, a downstream end of an annular member defining the radially outer extremity of the outer air swirler passage being frustoconical, the downstream end of the annular member defining the radially outer extremity of the outer air swirler passage being convergent in a downstream direction, the fuel supply passage being arranged to supply a first portion of the fuel into the inner air swirler passage and to supply a second portion of the fuel into the outer air swirler passage, the method comprising supplying a first portion of the fuel to a pre-filming surface in the inner air swirler passage and supplying a second portion of the fuel to a pre-filming surface in the outer air swirler passage.

Note that the inner air swirler passage and the outer air swirler passage being immediately adjacent each other means there are no other air swirler passages arranged coaxially between the inner air swirler passage and the outer air swirler passage.

The fuel injector may be a lean burn fuel injector the fuel feed arm having a single pilot fuel supply passage extending there-through and a single main fuel supply passage extending there-through, the fuel injector head having a coaxial arrangement of an inner pilot air-blast fuel injector and an outer main air-blast fuel injector, the outer main air-blast fuel injector being arranged coaxially radially outwardly of the inner pilot air-blast fuel injector, the inner pilot air-blast fuel injector comprising, in order radially outwardly, a coaxial arrangement of a pilot inner air swirler passage and a pilot outer air swirler passage, the outer main air-blast fuel injector comprising, in order radially outwardly, a coaxial arrangement of a main inner air swirler passage and a main outer air swirler passage, the method comprising supplying a first portion of the pilot fuel to a pre-filming surface in the pilot inner air swirler passage and supplying a second portion of the pilot fuel to a pre-filming surface in the pilot outer air swirler passage in a first mode of operation and supplying a first portion of the pilot fuel to the pre-filming surface in the pilot inner air swirler passage, supplying a second portion of the pilot fuel to the pre-filming surface in the pilot outer air swirler passage and supplying main fuel to a pre-filming surface in the main inner air swirler passage or to a pre-filming surface in the main outer air swirler passage in a second mode of operation.

The fuel injector may be a lean burn fuel injector the fuel feed arm having a single pilot fuel supply passage extending there-through and a single main fuel supply passage extending there-through, the fuel injector head having a coaxial arrangement of an inner pilot air-blast fuel injector and an outer main air-blast fuel injector, the outer main air-blast fuel injector being arranged coaxially radially outwardly of the inner pilot air-blast fuel injector, the inner pilot air-blast fuel injector comprising, in order radially outwardly, a coaxial arrangement of a pilot inner air swirler passage and a pilot outer air swirler passage, the outer main air-blast fuel injector comprising, in order radially outwardly, a coaxial arrangement of a main inner air swirler passage and a main outer air swirler passage, the method comprising supplying the pilot fuel to the a pre-filming surface in the pilot inner air swirler passage or to a pre-filming surface in the pilot outer air swirler passage in a first mode of operation and supplying the pilot fuel to the pre-filming surface in the pilot inner air swirler passage or to the pre-filming surface in the pilot outer air swirler passage, supplying a first portion of the main fuel to a pre-filming surface in the main inner air swirler passage and supplying a second portion of the main fuel to a pre-filming surface in the outer air swirler passage in a second mode of operation.

The fuel injector may be a lean burn fuel injector the fuel feed arm having a single pilot fuel supply passage extending there-through and a single main fuel supply passage extending there-through, the fuel injector head having a coaxial arrangement of an inner pilot air-blast fuel injector and an outer main air-blast fuel injector, the outer main air-blast fuel injector being arranged coaxially radially outwardly of the inner pilot air-blast fuel injector, the inner pilot air-blast fuel injector comprising, in order radially outwardly, a coaxial arrangement of a pilot inner air swirler passage and a pilot outer air swirler passage, the outer main air-blast fuel injector comprising, in order radially outwardly, a coaxial arrangement of a main inner air swirler passage and a main outer air swirler passage, the method comprising supplying a first portion of the pilot fuel to a pre-filming surface in the pilot inner air swirler passage and supplying a second portion of the pilot fuel to a pre-filming surface in the pilot outer air swirler passage in a first mode of operation and supplying a first portion of the pilot fuel to the pre-filming surface in the pilot inner air swirler passage, supplying a second portion of the pilot fuel to the pre-filming surface in the pilot outer air swirler passage, supplying a first portion of the main fuel to a pre-filming surface in the main inner air swirler passage and supplying a second portion of the main fuel to a pre-filming surface in the outer air swirler passage in a second mode of operation.

The mass flow of the fuel to the inner air swirler passage to the mass flow of the fuel to the outer air swirler passage may be between 10:1 and 1:10.

The mass flow of the fuel to the inner air swirler passage to the mass flow of the fuel to the pilot outer air swirler passage may be 1:1.

According to a third aspect of the present disclosure there is provided a lean burn fuel injector comprising a fuel feed arm and a fuel injector head, the fuel feed arm having a single pilot fuel supply passage extending there-through and a single main fuel supply passage extending there-through, the fuel injector head having a coaxial arrangement of an inner pilot air-blast fuel injector and an outer main air-blast fuel injector, the outer main air-blast fuel injector being arranged coaxially radially outwardly of the inner pilot air-blast fuel injector, the inner pilot air-blast fuel injector comprising, in order radially outwardly, a coaxial arrangement of a pilot inner air swirler passage and a pilot outer air swirler passage, the pilot inner air swirler passage and the pilot outer air swirler passage being arranged immediately adjacent each other, a downstream end of an annular member defining the radially outer extremity of the pilot outer air swirler passage being frustoconical, the downstream end of the annular member defining the radially outer extremity of the pilot outer air swirler passage being convergent in a downstream direction, the outer main air-blast fuel injector comprising, in order radially outwardly, a coaxial arrangement of a main inner air swirler passage and a main outer air swirler passage, the main inner air swirler passage and the main outer air swirler passage being arranged immediately adjacent each other, the single main fuel supply passage being arranged to supply main fuel to the main inner air swirler passage or the main outer air swirler passage, the single pilot fuel supply passage being arranged to supply pilot fuel into the pilot inner air swirler passage and to the pilot outer air swirler passage, the single main fuel supply passage being arranged to supply main fuel onto a pre-filming surface in the main inner air swirler passage or the single main fuel supply passage being arranged to supply pilot fuel onto a pre-filming surface in the main outer air swirler passage, the single pilot fuel supply passage being arranged to supply pilot fuel onto a pre-filming surface in the pilot inner air swirler passage, the single pilot fuel passage being arranged to supply pilot fuel onto a pre-filming surface in the pilot outer air swirler passage.

A pilot fuel passage may be arranged coaxially between the pilot inner air swirler passage and the pilot outer air swirler passage.

The single pilot fuel supply passage may be arranged to supply pilot fuel to the pilot fuel passage.

The pilot fuel passage may be arranged to supply the pilot fuel into the pilot inner air swirler passage and into the pilot outer air swirler passage.

The pilot fuel passage may be arranged to supply pilot fuel onto a pre-filming surface in the pilot inner air swirler passage.

The pilot fuel passage may be arranged to supply pilot fuel onto a pre-filming surface in the pilot outer air swirler passage.

The pilot fuel passage may have a fuel swirler to swirl the pilot fuel supplied onto the pre-filming surface in the pilot inner air swirler passage.

The pilot fuel passage may have a fuel swirler to swirl the pilot fuel supplied onto the pre-filming surface in the pilot outer air swirler passage.

A second pilot fuel passage may be arranged coaxially between the pilot outer air swirler passage and the main inner air swirler passage.

The single pilot fuel supply passage may be arranged to supply pilot fuel to the second pilot fuel passage.

The pilot fuel passage may be arranged to supply the pilot fuel into the pilot inner air swirler passage and the second pilot fuel passage may be arranged to supply pilot fuel into the pilot outer air swirler passage.

The pilot fuel passage may be arranged to supply pilot fuel onto a pre-filming surface in the pilot inner air swirler passage.

The second pilot fuel passage may be arranged to supply pilot fuel onto a pre-filming surface in the pilot outer air swirler passage.

The pilot fuel passage may have a fuel swirler to swirl the pilot fuel supplied onto the pre-filming surface in the pilot inner air swirler passage.

The second pilot fuel passage may have a fuel swirler to swirl the pilot fuel supplied onto the pre-filming surface in the pilot outer air swirler passage.

The pilot inner air swirler passage and the pilot outer air swirler passage may be arranged to swirl the air in opposite directions.

The number of points at which the pilot fuel is supplied into the pilot outer air swirler passage may be the same as the number of swirl vanes in the pilot outer air swirler passage.

The pilot fuel may be supplied into the pilot outer air swirler passage through an annular slot. The pilot fuel may be supplied into the pilot inner air swirler passage through an annular slot.

The points at which the pilot fuel is supplied into the pilot outer air swirler passage may be located adjacent the leading edge of the swirl vanes, adjacent the trailing edge of the swirl vanes or between the leading edge of the swirl vanes and the trailing edge of the swirl vanes.

The points at which the pilot fuel is supplied into the pilot outer air swirler passage may be aligned circumferentially with the leading edge of the swirl vanes, aligned circumferentially with the trailing edge of the swirl vanes or arranged circumferentially between the leading edge of the swirl vanes and the trailing edge of the swirl vanes.

The points at which the pilot fuel is supplied into the pilot outer air swirler passage may be equi-circumferentially spaced.

The mass flow of the pilot fuel to the pilot inner air swirler passage to the mass flow of the pilot fuel to the pilot outer air swirler passage may be between 10:1 and 1:10.

The mass flow of the pilot fuel to the pilot inner air swirler passage to the mass flow of the pilot fuel to the pilot outer air swirler passage may be 1:1.

The main fuel may be supplied into the main inner air swirler passage through an annular slot. The main fuel may be supplied into the main outer air swirler passage through an annular slot.

The main inner air swirler passage and the main outer air swirler passage may be arranged to swirl the air in opposite directions.

A main fuel passage may be arranged coaxially between the main inner air swirler passage and the main outer air swirler passage.

An intermediate air swirler passage may be sandwiched between the pilot outer air swirler passage of the inner pilot air-blast fuel injector and the main inner air swirler passage of the outer main air-blast fuel injector.

According to a fourth aspect of the present disclosure there is provided a method of operating a lean burn fuel injector, the lean burn fuel injector comprising a fuel feed arm and a fuel injector head, the fuel feed arm having a single pilot fuel supply passage extending there-through and a single main fuel supply passage extending there-through, the fuel injector head having a coaxial arrangement of an inner pilot air-blast fuel injector and an outer main air-blast fuel injector, the outer main air-blast fuel injector being arranged coaxially radially outwardly of the inner pilot air-blast fuel injector, the inner pilot air-blast fuel injector comprising, in order radially outwardly, a coaxial arrangement of a pilot inner air swirler passage and a pilot outer air swirler passage, the pilot inner air swirler passage and the pilot outer air swirler passage being arranged immediately adjacent each other, a downstream end of an annular member defining the radially outer extremity of the pilot outer air swirler passage being frustoconical, the downstream end of the annular member defining the radially outer extremity of the pilot outer air swirler passage being convergent in a downstream direction, the outer main air-blast fuel injector comprising, in order radially outwardly, a coaxial arrangement of a main inner air swirler passage and a main outer air swirler passage, the main inner air swirler passage and the main outer air swirler passage being arranged immediately adjacent each other, the single main fuel supply passage being arranged to supply main fuel to the main inner air swirler passage or the main outer air swirler passage, the single pilot fuel supply passage being arranged to supply pilot fuel into the pilot inner air swirler passage and to the pilot outer air swirler passage, the method comprising supplying pilot fuel to a pre-filming surface in the pilot inner air swirler passage and supplying pilot fuel to a pre-filming surface in the pilot outer air swirler passage in a first mode of operation and supplying pilot fuel to the pre-filming surface in the pilot inner air swirler passage, supplying pilot fuel to the pre-filming surface in the pilot outer air swirler passage and supplying main fuel to a pre-filming surface in the main inner air swirler passage or to a pre-filming surface in the main outer air swirler passage.

The mass flow of the pilot fuel to the pilot inner air swirler passage to the mass flow of the pilot fuel to the pilot outer air swirler passage may be between 10:1 and 1:10.

The mass flow of the pilot fuel to the pilot inner air swirler passage to the mass flow of the pilot fuel to the pilot outer air swirler passage may be 1:1.

The fuel injector may be provided in a combustion chamber. The fuel injector may be provided in a gas turbine engine.

The skilled person will appreciate that except where mutually exclusive, a feature described in relation to any one of the above aspects may be applied mutatis mutandis to any other aspect. Furthermore except where mutually exclusive any feature described herein may be applied to any aspect and/or combined with any other feature described herein.

DESCRIPTION OF THE DRAWINGS

Embodiments will now be described by way of example only, with reference to the Figures, in which:

FIG. 1 is a cross-sectional side view of a gas turbine engine.

FIG. 2 is an enlarged cross-sectional view of an annular combustion chamber of the gas turbine engine.

FIG. 3 is a further enlarged cross-section view of a lean burn fuel injector.

FIG. 4 is a further enlarged schematic cross-sectional view of the pilot fuel injector of the lean burn fuel injector shown in FIG. 3.

FIG. 5 is a schematic view of the pilot fuel injector shown in FIG. 4.

FIG. 6 is an alternative further enlarged schematic cross-sectional view of the pilot fuel injector of the lean burn fuel injector shown in FIG. 3.

FIG. 7 is another alternative further enlarged schematic cross-sectional view of the pilot fuel injector of the lean burn fuel injector shown in FIG. 3.

DETAILED DESCRIPTION

With reference to FIG. 1, a gas turbine engine is generally indicated at 10, having a principal and rotational axis X. The engine 10 comprises, in axial flow series, an air intake 11, a propulsive fan 12, an intermediate pressure compressor 13, a high-pressure compressor 14, combustion equipment 15, a high-pressure turbine 16, an intermediate pressure turbine 17, a low-pressure turbine 18 and an exhaust nozzle 19. A nacelle 21 generally surrounds the engine 10 and defines both the intake 11 and the exhaust nozzle 19.

The gas turbine engine 10 works in the conventional manner so that air entering the intake 11 is accelerated by the fan 12 to produce two air flows: a first air flow into the intermediate pressure compressor 13 and a second air flow which passes through a bypass duct 22 to provide propulsive thrust. The intermediate pressure compressor 13 compresses the air flow directed into it before delivering that air to the high pressure compressor 14 where further compression takes place.

The compressed air exhausted from the high-pressure compressor 14 is directed into the combustion equipment 15 where it is mixed with fuel and the mixture combusted. The resultant hot combustion products then expand through, and thereby drive the high, intermediate and low-pressure turbines 16, 17, 18 before being exhausted through the nozzle 19 to provide additional propulsive thrust. The high, intermediate and low pressure turbines 17, 18 and 19 respectively drive the high pressure compressor 15, intermediate pressure compressor 14 and the fan 13 respectively, each by a suitable interconnecting shaft 26, 28 and 30 respectively.

Other gas turbine engines to which the present disclosure may be applied may have alternative configurations. By way of example such engines may have an alternative number of interconnecting shafts (e.g. two) and/or an alternative number of compressors and/or turbines. Further the engine may comprise a gearbox provided in the drive train from a turbine to a compressor and/or fan.

The combustion chamber 15 is shown more clearly in FIG. 2. The combustion chamber 15 is an annular combustion chamber and comprises an inner annular wall 32, an outer annular wall 34 and an upstream wall 36. The upstream end wall 36 has a plurality of circumferentially spaced apertures, for example equi-circumferentially spaced apertures, 38. The combustion chamber 15 is surrounded by a combustion chamber casing 40 and the combustion chamber casing 40 has a plurality of circumferentially spaced apertures 42. The combustion chamber 15 also has a plurality of fuel injectors 44 and each fuel injector 40 extends radially through a corresponding one of the apertures 42 in the combustion chamber casing 40 and locates in a corresponding one of the apertures 38 in the upstream end wall 36 of the combustion chamber 15 to supply fuel into the combustion chamber 15.

A fuel injector 44 according to the present disclosure is shown more clearly in FIGS. 3 to 5. The fuel injector 44 comprises a fuel feed arm 46 and a fuel injector head 48. The fuel feed arm 46 has a single first internal fuel passage, a single pilot fuel supply passage, 50 for the supply of pilot fuel to the fuel injector head 48 and a single second internal fuel passage, a single main fuel supply passage, 52 for the supply of main fuel to the fuel injector head 48. The fuel injector head 48 has an axis Y and the fuel feed arm 46 extends generally radially with respect to the axis Y of the fuel injector head 48 and also generally radially with respect to the axis X of the turbofan gas turbine engine 10. The axis Y of each fuel injector head 48 is generally aligned with the axis of the corresponding aperture 38 in the upstream end wall 36 of the combustion chamber 15.

The fuel injector head 48 has a coaxial arrangement of an inner pilot air-blast fuel injector 54 and an outer main air-blast fuel injector 56. The inner pilot air-blast fuel injector 54 comprises, in order radially outwardly, a coaxial arrangement of a pilot inner air swirler passage 60, a pilot fuel passage 62 and a pilot outer air swirler passage 64. The outer main air-blast fuel injector 56 comprises, in order radially outwardly, a coaxial arrangement of a main inner air swirler passage 68, a main fuel passage 70 and a main outer air swirler passage 72. An intermediate air swirler passage 66 is sandwiched between the pilot outer air swirler passage 64 of the inner pilot air-blast fuel injector 54 and the main inner air swirler passage 68 of the outer main air-blast fuel injector 56.

The fuel injector head 48 comprises a first generally cylindrical member 74, a second generally annular member 76 spaced coaxially around the first member 74 and a third generally annular member 78 spaced coaxially around the second annular member 76. A plurality of circumferentially spaced swirl vanes 80 extend radially between the first member 74 and the second annular member 76 to form a first air swirler 81. The second annular member 76 has a greater axial length than the first member 74 and the first member 74 is positioned at an upstream end 76A of the second annular member 76 and a generally annular duct 60A is defined between the first member 74 and the second annular member 76 and the swirl vanes 80 extend radially across the annular duct 60A. A generally cylindrical duct 60B is defined radially within the second annular member 76 at a position downstream of the first member 74. The pilot inner air swirler passage 60 comprises the annular duct 60A and the cylindrical duct 60B.

The second annular member 76 has one or more internal fuel passages 62 which are arranged to receive fuel from the single first internal fuel passage, the single pilot fuel supply passage, 50 in the fuel feed arm 46. The one or more fuel passages 62 are arranged to supply fuel to a fuel swirler 84 which supplies a film of fuel onto a radially inner surface, a pre-filming surface, 86 at a downstream end 76B of the second annular member 76. A plurality of circumferentially spaced swirl vanes 88 extend radially between the second annular member 76 and the third annular member 78 to form a second air swirler 89. The second annular member 76 has a greater axial length than the third annular member 78 and the third annular member 78 is positioned at the downstream end 76B of the second annular member 76 and a generally annular duct 64A is defined between the second annular member 76 and the third annular member 78 and the swirl vanes 88 extend across the annular duct 64A. The pilot outer air swirler passage 64 comprises the annular duct 64A. The one or more fuel passages 62 are also arranged to supply fuel to a fuel swirler 90 which supplies a film of fuel onto a radially outer surface, a pre-filming surface, 92 at a downstream end 76B of the second annular member 76. The pre-filming surface 92 is located axially downstream of the swirl vanes 88 of the second swirler 89. The radially outer surface, the pre-filming surface, 92 is a frustoconical surface which converges in a downstream direction.

The downstream end 78B of the third annular member 78 is conical and is convergent in a downstream direction. The downstream end 78B of the third annular member 78 is downstream of the downstream end 76B of the second annular member 76 and the downstream end 76B of the second annular member 76 is downstream of the downstream end 74B of the first member 74. In operation the pilot fuel supplied by internal fuel passages 62 and fuel swirler 84 onto the radially inner surface 86 of the second annular member 76 is atomised by swirling flows of air from the swirl vanes 80 and 88 of the first and second air swirlers 81 and 89 respectively. In operation the pilot fuel supplied by internal fuel passages 62 and fuel swirler 90 onto the radially outer surface 92 of the second annular member 76 is atomised by swirling flows of air from the swirl vanes 80 and 88 of the first and second air swirlers 81 and 89 respectively. The pilot inner air swirler passage 60 and the pilot outer air swirler passage 64 are arranged to swirl the air in opposite directions. Alternatively, the pilot inner air swirler passage 60 and the pilot outer air swirler passage 64 may be arranged to swirl the air in the same direction.

The fuel injector head 48 also comprises a fourth generally annular member 94 spaced coaxially around the third annular member 78, a fifth generally annular member 96 spaced coaxially around the fourth annular member 94 and a sixth generally annular member 98 spaced coaxially around the fifth annular member 96. A plurality of circumferentially spaced swirl vanes 100 extend radially between the fourth annular member 94 and the fifth annular member 96 to form a third air swirler 101. The fifth annular member 96 has a greater axial length than the fourth annular member 94 and the fourth annular member 94 is positioned at the downstream end 96B of the fifth annular member 96 and a generally annular duct 68A is defined between the fourth annular member 94 and the fifth annular member 96 and the swirl vanes 100 extend across the annular duct 68A. The main inner air swirler passage 68 comprises the annular duct 68A. The fifth annular member 96 has one or more internal fuel passages 70 which are arranged to receive fuel from the single second internal fuel passage 52 in the fuel feed arm 46. The one or more fuel passages 70 are arranged to supply fuel to a fuel swirler (not shown) which supplies a film of fuel through outlet 70A onto the radially inner surface 102 at the downstream end 96B of the fifth annular member 96. The radially inner surface 102 of the fifth annular member 96 is a pre-filming surface. A plurality of circumferentially spaced swirl vanes 104 extend radially between the fifth annular member 96 and the sixth annular member 98 to form a fourth air swirler 105. A generally annular duct 72A is defined between the downstream end 96B of the fifth annular member 96 and the downstream end 98B of the sixth annular member 98 and the swirl vanes 104 extend across the annular duct 72A. The main outer air swirler passage 72 comprises the annular duct 72A. The downstream end 94B of the fourth annular member 94 is conical and is divergent in a downstream direction. In operation the main fuel supplied by internal fuel passages 70, fuel swirler and outlet 70A onto the radially inner surface 102 of the fifth annular member 96 is atomised by swirling flows of air from the swirl vanes 100 and 104 of the third and fourth air swirlers 101 and 105 respectively. The main inner air swirler passage 68 and the main outer air swirler passage 72 are arranged to swirl the air in opposite directions. Alternatively, the main inner air swirler passage 68 and the main outer air swirler passage 72 may be arranged to swirl the air in the same direction.

The fuel injector head 48 also comprises a plurality of circumferentially spaced swirl vanes which extend radially between the third annular member 78 and the fourth annular member 94 to form a fifth air swirler. An annular duct is defined between the third annular member 78 and the fourth annular member 94. The intermediate air swirler passage 66 comprises the annular duct. The intermediate air swirler passage 66 is sandwiched between the pilot outer air swirler passage 64 of the inner pilot air-blast fuel injector 54 and the main inner air swirler passage 68 of the outer main air-blast fuel injector 56. In operation the swirl vanes of the fifth air swirler provide a swirling flow of air over the radially inner surface of the fourth annular member 94.

As discussed above, in operation, the pilot fuel supplied from the single first internal fuel passage 50 in the fuel feed arm 46 to the at least one internal fuel passage 62 is split and a first portion of the pilot fuel is supplied onto the radially inner surface 86 of the second annular member 76 and a second portion of the pilot fuel is supplied onto the radially outer surface 92 of the second annular member 76. The pilot fuel and air mixture from the pilot inner air swirler passage 60 tends to flow directly downstream along the centre line of the annular combustion chamber 15. The main fuel and air mixture from the main inner air swirler passage 68 tends to flow radially inwardly and radially outwardly towards the inner and outer annular walls 32 and 34 respectively of the annular combustion chamber 15.

As shown in FIG. 4 there are six points at which the pilot fuel is supplied into the pilot inner air swirler passage 62 and twelve points at which the pilot fuel is supplied into the pilot outer air swirler passage 64. The number of points at which the pilot fuel is supplied into the pilot outer air swirler passage 64 may be the same as the number of swirl vanes in the pilot outer air swirler passage 64. The points at which the pilot fuel is supplied into the pilot outer air swirler passage 64 may be located adjacent the leading edge of the swirl vanes, adjacent the trailing edge of the swirl vanes or between the leading edge of the swirl vanes and the trailing edge of the swirl vanes. The points at which the pilot fuel is supplied into the pilot outer air swirler passage may be aligned circumferentially with the leading edge of the swirl vanes, aligned circumferentially with the trailing edge of the swirl vanes or arranged circumferentially between the leading edge of the swirl vanes and the trailing edge of the swirl vanes. The points at which the pilot fuel is supplied into the pilot outer air swirler passage 64 may be equi-circumferentially spaced. The number of points at which, and their area, the pilot fuel is supplied into the pilot inner air swirler passage 60 and the number of points at which, and their area, the pilot fuel is supplied into the pilot outer air swirler passage 64 are selected such that the mass flow of the pilot fuel to the pilot inner air swirler passage 60 to the mass flow of the pilot fuel to the pilot outer air swirler passage 64 may be between 10:1 and 1:10 and may be 1:1.

The fuel and air mixture from the pilot outer air swirler passage 64 provides a recirculating feature to couple the flows from the pilot inner air swirler passage 60 and the main inner air swirler passage 68. Thus, some of the pilot fuel is supplied into the recirculating feature and this pilot fuel takes part in the pilot combustion process and the main combustion process without compromising the original design function. The supply of some of the pilot fuel into the pilot outer air swirler passage 64 provides the possibility of greater interaction of the pilot fuel and the air to provide better mixing of the pilot fuel and air so that the atomised pilot fuel particles have a lower SMD (Sauter Mean Diameter) and so that there is a reduction in emissions, e.g. smoke and unburned hydrocarbons. Furthermore, the supply of some of the pilot fuel into the pilot outer air swirler passage 64 stabilises the pilot combustion zone where pilot combustion process occurs and the main combustion zone where main combustion process occurs. The supply of pilot fuel into the pilot outer air swirler passage 64 may be tailored to reduce circumferential variation in the fuel and air mixture.

An alternative pilot fuel injector 54A of the lean burn fuel injector 44 is shown in FIG. 6 and is substantially the same as that shown in FIG. 4 and like parts are denoted by like numbers. The pilot fuel injector 54A shown in FIG. 6 differs in that the one or more fuel passages 62 are arranged to supply fuel to a fuel swirler 90A which supplies a film of fuel onto a radially outer surface, a pre-filming surface, 92A of the second annular member 76. The pre-filming surface 92A is located at a position axially upstream of that shown in FIG. 4 and in general the pre-filming surface 92A may be located at any suitable axial position in the pilot outer air swirler passage 64. The pre-filming surface 92A is located axially downstream of the swirl vanes 88 of the second swirler 89. The radially outer surface, the pre-filming surface, 92A is a cylindrical surface.

Another alternative pilot fuel injector 54B of the lean burn fuel injector 44 is shown in FIG. 7 and is substantially the same as that shown in FIG. 4 and like parts are denoted by like numbers. The pilot fuel injector 54B shown in FIG. 7 differs in that the one or more fuel passages 62B are arranged to supply fuel to a fuel swirler 90B which supplies a film of fuel onto a radially inner surface, a pre-filming surface, 92B of the third annular member 78. The one or more fuel passages 62B and the fuel swirler 90B are arranged in the third annular member 78 and the single pilot fuel supply passage 50 in the fuel feed arm 46 is arranged to supply pilot fuel to the one or more fuel passages 62B.

The pre-filming surface 92B is located at a position axially upstream of that shown in FIG. 4 and in general the pre-filming surface 92B may be located at any suitable axial position in the pilot outer air swirler passage 64. The pre-filming surface 92B is located axially downstream of the swirl vanes 88 of the second swirler 89. The radially inner surface, the pre-filming surface, 92B is a cylindrical surface.

A further alternative fuel injector arrangement to the present disclosure is shown in FIG. 3. The main fuel passage 70 may have an outlet 70B to supply fuel onto the radially outer surface 106 of the fifth annular member 96. The radially outer surface 106 is a pre-filming surface. Thus, in operation, the pilot fuel supplied from the single first internal fuel passage 50 in the fuel feed arm 46 to the at least one internal fuel passage 62 is split and a first portion of the pilot fuel is supplied onto the radially inner surface 86 of the second annular member 76 and a second portion of the pilot fuel is supplied onto the radially outer surface 92 of the second annular member 76. The main fuel supplied from the single second internal fuel passage 52 in the fuel feed arm 46 to the at least one internal fuel passage 70 is split and a first portion of the main fuel is supplied onto the radially inner surface 102 of the fifth annular member 96 and a second portion of the main fuel is onto the radially outer surface 106 of the fifth annular member 96. The radially inner surface 102 and the radially outer surface 106 of the fifth annular member 96 are pre-filming surfaces. The pilot fuel and air mixture from the pilot inner air swirler passage 60 tends to flow directly downstream along the centre line of the annular combustion chamber 15. The main fuel and air mixture from the main inner air swirler passage 68 tends to flow radially inwardly and radially outwardly towards the inner and outer annular walls 32 and 34 respectively of the annular combustion chamber 15.

The supply of some of the pilot fuel into the main outer air swirler passage provides the possibility of greater interaction of the main fuel and the air to provide better mixing of the main fuel and air so that the atomised main fuel particles have a lower SMD (Sauter Mean Diameter) and so that there is a reduction in emissions, e.g. smoke and unburned hydrocarbons.

In each of the above lean burn fuel injector arrangement the pilot fuel may be supplied into the pilot outer air swirler passage through an annular slot. In each of the above lean burn fuel injector arrangements the pilot fuel may be supplied into the pilot inner air swirler passage through an annular slot. In each of the above lean burn fuel injector arrangements the main fuel may be supplied into the main inner air swirler passage through an annular slot. In each of the above lean burn fuel injector arrangements the main fuel may be supplied into the main outer air swirler passage through an annular slot.

The present disclosure is also applicable to a rich burn fuel injector which comprises an inner air swirler passage and an outer air swirler passage in which a first portion of the fuel is supplied into the inner air swirler passage and a second portion of the fuel is supplied into the outer air swirler passage and to a rich burn fuel injector which comprises an inner air swirler passage, an outer air swirler passage and an additional air swirler passage arranged coaxially around the outer air swirler passage in which a first portion of the fuel is supplied into the inner air swirler passage and a second portion of the fuel is supplied into the outer air swirler passage. The fuel supply passage is arranged to supply fuel onto a pre-filming surface in the inner air swirler passage, the fuel supply passage being arranged to supply fuel onto a pre-filming surface in the outer air swirler passage. The outer air swirler passage in this arrangement is an intermediate air swirler passage between to two coaxial air swirler passages. A downstream end of an annular member defining the radially outer extremity of the outer air swirler passage is frustoconical, the downstream end of the annular member defining the radially outer extremity of the outer air swirler passage is convergent in a downstream direction. A downstream end of an annular member defining the radially outer extremity of the additional air swirler passage is frustoconical, the downstream end of the annular member defining the radially outer extremity of the outer air swirler passage is convergent in a downstream direction. The supply of some of the fuel into the outer air swirler passage provides the possibility of greater interaction of the main fuel and the air to provide better mixing of the main fuel and air so that the atomised main fuel particles have a lower SMD (Sauter Mean Diameter) and so that there is a reduction in emissions, e.g. smoke and unburned hydrocarbons.

In each of the above rich burn fuel injector arrangements the fuel may be supplied into the inner air swirler passage through an annular slot. In each of the above rich burn fuel injector arrangements the fuel may be supplied into the outer air swirler passage through an annular slot.

It will be understood that the invention is not limited to the embodiments above-described and various modifications and improvements can be made without departing from the concepts described herein. Except where mutually exclusive, any of the features may be employed separately or in combination with any other features and the disclosure extends to and includes all combinations and sub-combinations of one or more features described herein. 

1. A fuel injector comprising a fuel feed arm and a fuel injector head, the fuel feed arm having a fuel supply passage extending there-through, the fuel injector head having an air-blast fuel injector, the air-blast fuel injector comprising, in order radially outwardly, a coaxial arrangement of an inner air swirler passage and an outer air swirler passage, the inner air swirler passage and the outer air swirler passage being arranged immediately adjacent each other, an annular member defining the radially outer extremity of the outer air swirler passage, a downstream end of the annular member defining the radially outer extremity of the outer air swirler passage being frustoconical, the downstream end of the annular member defining the radially outer extremity of the outer air swirler passage being convergent in a downstream direction, the fuel supply passage being arranged to supply a first portion of the fuel into the inner air swirler passage and to supply a second portion of the fuel into the outer air swirler passage, the inner air swirler passage having a pre-filming surface, the outer air swirler passage having a pre-filming surface, the fuel supply passage being arranged to supply fuel onto the pre-filming surface in the inner air swirler passage, the fuel supply passage being arranged to supply fuel onto the pre-filming surface in the outer air swirler passage.
 2. A fuel injector as claimed in claim 1, wherein the fuel injector is a rich burn fuel injector, the fuel injector having an additional air swirler passage arranged coaxially around the outer air swirler passage.
 3. A fuel injector as claimed in claim 1, wherein the fuel injector is a lean burn fuel injector, the fuel feed arm having a single pilot fuel supply passage extending there-through and a single main fuel supply passage extending there-through, the fuel injector head having a coaxial arrangement of an inner pilot air-blast fuel injector and an outer main air-blast fuel injector, the outer main air-blast fuel injector being arranged coaxially radially outwardly of the inner pilot air-blast fuel injector, the inner pilot air-blast fuel injector comprising, in order radially outwardly, a coaxial arrangement of a pilot inner air swirler passage and a pilot outer air swirler passage, the outer main air-blast fuel injector comprising, in order radially outwardly, a coaxial arrangement of a main inner air swirler passage and a main outer air swirler passage.
 4. A fuel injector as claimed in claim 3, wherein the single main fuel supply passage being arranged to supply main fuel to the main inner air swirler passage or the main outer air swirler passage, the single pilot fuel supply passage being arranged to supply a first portion of pilot fuel into the pilot inner air swirler passage and to supply a second portion of pilot fuel into the pilot outer air swirler passage, the single main fuel supply passage being arranged to supply main fuel onto a pre-filming surface in the main inner air swirler passage or the single main fuel supply passage being arranged to supply pilot fuel onto a pre-filming surface in the main outer air swirler passage, the single pilot fuel supply passage being arranged to supply pilot fuel onto a pre-filming surface in the pilot inner air swirler passage, the single pilot fuel supply passage being arranged to supply pilot fuel onto a pre-filming surface in the pilot outer air swirler passage.
 5. A fuel injector as claimed in claim 3, wherein the single pilot fuel supply passage being arranged to supply pilot fuel into the pilot inner air swirler passage or the pilot outer air swirler passage, the single main fuel supply passage being arranged to supply a first portion of the main fuel into the main inner air swirler passage and to supply a second portion of the main fuel into the main outer air swirler passage, the single main fuel supply passage being arranged to supply main fuel onto a pre-filming surface in the main inner air swirler passage, the single main fuel supply passage being arranged to supply pilot fuel onto a pre-filming surface in the main outer air swirler passage, the single pilot fuel supply passage being arranged to supply pilot fuel onto a pre-filming surface in the pilot inner air swirler passage or the single pilot fuel supply passage being arranged to supply pilot fuel onto a pre-filming surface in the pilot outer air swirler passage.
 6. A fuel injector as claimed in claim 3, wherein the single pilot fuel supply passage being arranged to supply a first portion of pilot fuel into the pilot inner air swirler passage and to supply a second portion of pilot fuel into the pilot outer air swirler passage and the single main fuel supply passage being arranged to supply a first portion of the main fuel into the main inner air swirler passage and to supply a second portion of the main fuel into the main outer air swirler passage, the single main fuel supply passage being arranged to supply main fuel onto a pre-filming surface in the main inner air swirler passage, the single main fuel supply passage being arranged to supply pilot fuel onto a pre-filming surface in the main outer air swirler passage, the single pilot fuel supply passage being arranged to supply pilot fuel onto a pre-filming surface in the pilot inner air swirler passage and the single pilot fuel supply passage being arranged to supply pilot fuel onto a pre-filming surface in the pilot outer air swirler passage.
 7. A fuel injector as claimed in claim 3, wherein a pilot fuel passage is arranged coaxially between the pilot inner air swirler passage and the pilot outer air swirler passage, the single pilot fuel supply passage is arranged to supply pilot fuel to the pilot fuel passage.
 8. A fuel injector as claimed in claim 7, wherein the pilot fuel passage is arranged to supply the pilot fuel into the pilot inner air swirler passage and into the pilot outer air swirler passage.
 9. A fuel injector as claimed in claim 7, wherein the pilot fuel passage has a fuel swirler to swirl the pilot fuel supplied onto the pre-filming surface in the pilot inner air swirler passage.
 10. A fuel injector as claimed in claim 7, wherein the pilot fuel passage has a fuel swirler to swirl the pilot fuel supplied onto the pre-filming surface in the pilot outer air swirler passage.
 11. A fuel injector as claimed in claim 7, wherein a second pilot fuel passage is arranged coaxially between the pilot outer air swirler passage and the main inner air swirler passage, the single pilot fuel supply passage is arranged to supply pilot fuel to the second pilot fuel passage, the pilot fuel passage is arranged to supply the pilot fuel into the pilot inner air swirler passage and the second pilot fuel passage is arranged to supply pilot fuel into the pilot outer air swirler passage.
 12. A fuel injector as claimed in claim 11, wherein the pilot fuel passage has a fuel swirler to swirl the pilot fuel supplied onto the pre-filming surface in the pilot inner air swirler passage.
 13. A fuel injector as claimed in claim 11, wherein the second pilot fuel passage has a fuel swirler to swirl the pilot fuel supplied onto the pre-filming surface in the pilot outer air swirler passage.
 14. A fuel injector as claimed in claim 3, wherein the pilot inner air swirler passage and the pilot outer air swirler passage are arranged to swirl the air in opposite directions.
 15. A fuel injector as claimed in claim 1, wherein the number of points at which the fuel is supplied into the outer air swirler passage is the same as the number of swirl vanes in the outer air swirler passage.
 16. A fuel injector as claimed in claim 15, wherein the points at which the pilot fuel is supplied into the outer air swirler passage are located adjacent the leading edge of the swirl vanes, adjacent the trailing edge of the swirl vanes or between the leading edge of the swirl vanes and the trailing edge of the swirl vanes.
 17. A fuel injector as claimed in claim 15, wherein the points at which the fuel is supplied into the outer air swirler passage are aligned circumferentially with the leading edge of the swirl vanes, aligned circumferentially with the trailing edge of the swirl vanes or arranged circumferentially between the leading edge of the swirl vanes and the trailing edge of the swirl vanes.
 18. A fuel injector as claimed in claim 3, wherein a main fuel passage is arranged coaxially between the main inner air swirler passage and the main outer air swirler passage.
 19. A fuel injector as claimed in claim 3, wherein an intermediate air swirler passage is sandwiched between the pilot outer air swirler passage of the inner pilot air-blast fuel injector and the main inner air swirler passage of the outer main air-blast fuel injector.
 20. A method of operating a fuel injector, the fuel injector comprising a fuel feed arm and a fuel injector head, the fuel feed arm having a fuel supply passage extending there-through, the fuel injector head having an air-blast fuel injector, the air-blast fuel injector comprising, in order radially outwardly, a coaxial arrangement of an inner air swirler passage and an outer air swirler passage, the inner air swirler passage and the outer air swirler passage being arranged immediately adjacent each other, an annular member defining he radially outer extremity of the outer air swirler passage, a downstream end of the annular member defining the radially outer extremity of the outer air swirler passage being frustoconical, the downstream end of the annular member defining the radially outer extremity of the outer air swirler passage being convergent in a downstream direction, the fuel supply passage being arranged to supply a first portion of the fuel into the inner air swirler passage and to supply a second portion of the fuel into the outer air swirler passage, the inner air swirler passage having a pre-filming surface, the outer air swirler passage having a pre-filming surface, the method comprising supplying a first portion of the fuel to the pre-filming surface in the inner air swirler passage and supplying a second portion of the fuel to the pre-filming surface in the outer air swirler passage.
 21. A method as claimed in claim 20 wherein the fuel injector is a lean burn fuel injector, the fuel feed arm having a single pilot fuel supply passage extending there-through and a single main fuel supply passage extending there-through, the fuel injector head having a coaxial arrangement of an inner pilot air-blast fuel injector and an outer main air-blast fuel injector, the outer main air-blast fuel injector being arranged coaxially radially outwardly of the inner pilot air-blast fuel injector, the inner pilot air-blast fuel injector comprising, in order radially outwardly, a coaxial arrangement of a pilot inner air swirler passage and a pilot outer air swirler passage, the outer main air-blast fuel injector comprising, in order radially outwardly, a coaxial arrangement of a main inner air swirler passage and a main outer air swirler passage, the method comprising supplying a first portion of the pilot fuel to a pre-filming surface in the pilot inner air swirler passage and supplying a second portion of the pilot fuel to a pre-filming surface in the pilot outer air swirler passage in a first mode of operation and supplying a first portion of the pilot fuel to the pre-filming surface in the pilot inner air swirler passage, supplying a second portion of the pilot fuel to the pre-filming surface in the pilot outer air swirler passage and supplying main fuel to a pre-filming surface in the main inner air swirler passage or to a pre-filming surface in the main outer air swirler passage in a second mode of operation.
 22. A method as claimed in claim 20 wherein the fuel injector is a lean burn fuel injector the fuel feed arm having a single pilot fuel supply passage extending there-through and a single main fuel supply passage extending there-through, the fuel injector head having a coaxial arrangement of an inner pilot air-blast fuel injector and an outer main air-blast fuel injector, the outer main air-blast fuel injector being arranged coaxially radially outwardly of the inner pilot air-blast fuel injector, the inner pilot air-blast fuel injector comprising, in order radially outwardly, a coaxial arrangement of a pilot inner air swirler passage and a pilot outer air swirler passage, the outer main air-blast fuel injector comprising, in order radially outwardly, a coaxial arrangement of a main inner air swirler passage and a main outer air swirler passage, the method comprising supplying the pilot fuel to a pre-filming surface in the pilot inner air swirler passage or to a pre-filming surface in the pilot outer air swirler passage in a first mode of operation and supplying the pilot fuel to the pre-filming surface in the pilot inner air swirler passage or to the pre-filming surface in the pilot outer air swirler passage, supplying a first portion of the main fuel to a pre-filming surface in the main inner air swirler passage and supplying a second portion of the main fuel to a pre-filming surface in the outer air swirler passage in a second mode of operation.
 23. A method as claimed in claim 20 wherein the fuel injector is a lean burn fuel injector the fuel feed arm having a single pilot fuel supply passage extending there-through and a single main fuel supply passage extending there-through, the fuel injector head having a coaxial arrangement of an inner pilot air-blast fuel injector and an outer main air-blast fuel injector, the outer main air-blast fuel injector being arranged coaxially radially outwardly of the inner pilot air-blast fuel injector, the inner pilot air-blast fuel injector comprising, in order radially outwardly, a coaxial arrangement of a pilot inner air swirler passage and a pilot outer air swirler passage, the outer main air-blast fuel injector comprising, in order radially outwardly, a coaxial arrangement of a main inner air swirler passage and a main outer air swirler passage, the method comprising supplying a first portion of the pilot fuel to a pre-filming surface in the pilot inner air swirler passage and supplying a second portion of the pilot fuel to a pre-filming surface in the pilot outer air swirler passage in a first mode of operation and supplying a first portion of the pilot fuel to the pre-filming surface in the pilot inner air swirler passage, supplying a second portion of the pilot fuel to the pre-filming surface in the pilot outer air swirler passage, supplying a first portion of the main fuel to a pre-filming surface in the main inner air swirler passage and supplying a second portion of the main fuel to a pre-filming surface in the outer air swirler passage in a second mode of operation.
 24. A method as claimed in claim 20, wherein the mass flow of the fuel to the inner air swirler passage to the mass flow of the fuel to the outer air swirler passage is between 10:1 and 1:10.
 25. A method as claimed in claim 20, wherein the mass flow of the fuel to the inner air swirler passage to the mass flow of the fuel to the pilot outer air swirler passage is 1:1. 